The reduction of CO2 emission is one of the greatest concerns in combating the catastrophic xe2x80x9cglobal warmingxe2x80x9d trend. As a result, the world puts much emphasis on the exploitation of xe2x80x9cclean energyxe2x80x9d with less or non-emission for both industrial and domestic uses. Natural gas (hereafter abbreviated as xe2x80x9cNGxe2x80x9d), as compared with coal and petroleum, is considered the most economic xe2x80x9ccleanxe2x80x9d fuel that is used on a large, industrial scale at present and in the near future. In addition, the discovery of huge amount of ocean-bed gas-hydrates increases the recoverable resources of NG substantially. It is expected that, in the long run, the global NG consumption may eventually exceeds all other fossil fuels.
NG is a mixture of hydrocarbon gases, consisting of mainly methane (C1) and a smaller fraction of heavier gaseous hydrocarbons (i.e., ethane, C2; propane, C3; butane, C4; pentane and higher, C5+; sometimes C3+ is called xe2x80x9clight oilxe2x80x9d as a whole. However, the economic values of these higher hydrocarbon components, when separated and sold as chemical feedstock, are usually much higher than burnt as a fuel. A number of NG processing plants, therefore, have been constructed to extract these valuable materials.
The state-of-the-art NG processing plants generally work on a cryogenic process for efficiently separating the higher hydrocarbon gases In this process, a huge volume of NG is cooled down by expansion to a very low cryogenic temperature around xe2x88x92150xc2x0 F. Such a process is extremely energy-consuming, and the facility usually comprises many pieces of expensive equipment, notably the molecular-sieve dehydrator, the multiple-flow finned-plate heat exchanger, and the turbo expander-compressor. High capital and operational costs are thus resulted. As a consequence, only a limited fraction of the NG could be processed before consumed as a fuel. Most of the valuable higher hydrocarbon contents was improperly used.
In the past two decades, a number of US patents have been granted in this field, for example, the 13 US patents entitled xe2x80x9chydrocarbon Processingxe2x80x9d presented by late Roy E. Campbell, et al., i.e., U.S. Pat. Nos. 4,140,504; 4,157,904; 4,171,964; 4,278,457; 4,854,955; 4,869,740; 4,889,545; 5,555,784; 5,568,737; 5,771,712; 5,881,569; 5,983,664; and 6,182,469. However, most of these patents only proposed some specific improvements to the same cryogenic process. No substantial break-through in NG processing technology has ever been proposed. A more efficient and cost-effective technology for NG procession, therefore, is desirable.
The recent developments in NG refrigeration dehydration technology, e.g., those presented in U.S. Pat. No. 5,664,426, xe2x80x9cRegenerative Gas Dehydrator;xe2x80x9d 1997, and U.S. Pat. No. 6,158,242, xe2x80x9cGas Dehydration Method and Apparatus,xe2x80x9d 2000, provided the basis of a break-through in the NG processing technology. These patents make possible to perform refrigeration dehydration and refrigeration absorption in a single unit.
Accordingly, it is an objective of the present invention to provide a comprehensive NG processor, based on the refrigeration dehydration and absorption technologies, for efficient and cost-effective comprehensive processing of NG. The said processor could simultaneously perform the removal of moisture and the recovery of the higher hydrocarbons (C2+) in a single piece of equipment, thus substantially reducing the capital and operational costs of the NG processing plant.
Another objective of the present invention is to provide an energy-saving comprehensive NG processor that, when processing high pressure NG, does not need external energy for refrigeration.
A further objective of the present invention is to provide a high-efficiency free-piston expander-compressor to provide the required refrigeration.
With regard to the above and other objectives, the present invention provides a comprehensive NG processor to simultaneously perform refrigeration dehydration and refrigeration absorption of higher hydrocarbon gases with maximum recovery rate at minimum energy consumption. The final product is a gaseous mixture enriched in higher hydrocarbons with minimum residual methane.
The said apparatus comprises the following major components: an integrated NG processor (hereafter abbreviated as xe2x80x9cprocessor) with a refrigeration dehydration section (hereafter abbreviated as xe2x80x9cdehydratorxe2x80x9d) and a refrigeration absorption section (hereafter abbreviated as xe2x80x9cabsorberxe2x80x9d); a heat-transport medium (hereafter abbreviated as xe2x80x9cmediumxe2x80x9d) cooler; an absorbent cooler; a fractional distiller; a gas-hydrate inhibitor (hereafter abbreviated as xe2x80x9cinhibitorxe2x80x9d) regenerator; and a refrigeration unit.
The principle of the operations of the comprehensive NG processor follows. The inlet moisture-laden NG, flowing upward from the bottom of the dehydrator, is cooled down to the desired dewpoint temperature by directly contacting a down-flowing, adequately dispersed low-temperature medium stream. The medium is an aqueous solution containing an inhibitor. The moisture in the inlet NG is condensed on the surface of the medium droplets. The medium, diluted with the condensates, is re-concentrated in an inhibitor regenerator and recycled. The dehydrated NG continues to flow upward into the absorber wherein the higher hydrocarbon gases are absorbed with a down-flowing, adequately dispersed low-temperature absorbent (e.g., heavy oil) stream. The light oil-laden absorbent (hereafter abbreviated as xe2x80x9crich oilxe2x80x9d) then enters the fractional distiller wherein the absorbed higher hydrocarbons is separated as the final product. The recovered absorbent is cooled in the absorbent cooler and recycled to the absorber of the processor. The processed NG, basically free from higher hydrocarbons (hereafter abbreviated as xe2x80x9clean NGxe2x80x9d), is re-heated and eventually delivered to the NG transportation pipeline. The refrigeration unit provides the required refrigeration for both medium cooler and absorbent cooler.
When the pressure of the inlet NG is sufficiently high, the required refrigeration could be provided with expanding the dehydrated high pressure NG. In such a xe2x80x9cself-refrigerationxe2x80x9d case, no external energy is required.
In case of the pressure difference between the inlet NG and the NG transportation pipeline is small, a high-efficiency free-piston NG expander-compressor is proposed in the present invention to provide the required self-refrigeration.